Rare earth fluoride additive for sintering aluminum nitride

ABSTRACT

An aluminum nitride ceramic body with a thermal conductivity of at least 0.5 W/cm·K at 25° C. is produced by shaping a particulate mixture of aluminum nitride powder and an additive selected from the group consisting of YF 3 , ScF 3 , LF 3  where L is La, Ce, Pr, Nd, Sm, Gd, Dy, Ho and Er and a mixture thereof into a compact and liquid phase sintering the compact.

This application is a continuation of application Ser. No. 887,504,filed July 21, 1986, abandoned.

The present invention relates to the use of a rare earth fluorideadditive to produce a liquid phase sintered ceramic aluminum nitridebody having a minimum thermal conductivity of 0.5 W/cm.·at 25° C. and aporosity of less than 10%.

The thermal conductivity of an aluminum nitride single crystal is astrong function of dissolved oxygen and decreases with an increase indissolved oxygen content. For example, the thermal conductivity ofaluminum nitride single crystal having 0.8 wt % dissolved oxygen isabout 0.8 W/cm·K whereas a suitably pure aluminum nitride singlecrystal, containing 300 ppm dissolved oxygen, has been measured to havea room temperature thermal conductivity of 2.8 W/cm·K.

Aluminum nitride powder has an affinity for oxygen, especially when itssurface is not covered by an oxide. There are usually three differentsources of oxygen in nominally pure AlN powder. Source #1 is discreteparticles of Al₂ O₃. Source #2 is an oxide coating, perhaps as Al₂ O₃,coating the AlN powder particles. Source #3 is oxygen in solution in theAlN lattice. The amount of oxygen present in the AlN lattice in AlNpowder will depend on the method of preparing the AlN powder. In thepresent invention, a rare earth fluoride is used to densify anddeoxidize the aluminum nitride. According to the present invention,aluminum nitride powder can be processed in air and still will besufficiently deoxidized in the sintering step to produce a ceramic bodyhaving a thermal conductivity of at least 0.5 W/cm·K at 25° C.

Briefly stated, the present process for producing a sinteredpolycrystalline aluminum nitride body having a porosity of less thanabout 10% by volume and a thermal conductivity of at least 0.5 W/cm·K at25° C. comprises forming a mixture comprised of aluminum nitride powdercontaining oxygen and a fluoride additive, said additive being a memberselected from the group consisting of YF.sub. 3 ranging from about 0.5%by weight to about 15% by weight, ScF.sub. 3 ranging from about 0.5% byweight to about 10% by weight, LF.sub. 3 where L is La, Ce, Pr, Nd, Sm,Gd, Dy, Ho and Er, ranging from about 1% by weight to about 20% byweight, and a mixture of said member ranging from greater than about0.5% by weight to less than about 20% by weight, said % by weight ofsaid additive being based on the amount of said aluminum nitride powder,shaping said mixture into a compact, and sintering said compact in anitrogen-containing nonoxidizing atmosphere at a minimum temperature ofat least about 1700° C. producing said polycrystalline body, saidminimum sintering temperature being sufficient to product said sinteredbody.

In the present process, the aluminum nitride powder can be of commercialor technical grade. Specifically, it should not contain any impuritieswhich would have a significantly deleterious effect on the desiredproperties of the resulting sintered product. The starting aluminumnitride powder used in the present process contains oxygen generallyranging in an amount up to about 4.4% by weight and usually ranging inamount from greater than about 1.0% by weight to less than about 4.0% byweight. Typically, commercially available aluminum nitride powdercontains from about 1.5 weight % to about 3 weight % of oxygen and suchpowders are most preferred on the basis of their substantially lowercost.

Generally, the present starting aluminum nitride powder has a specificsurface area which can range widely, and generally it ranges up to about10 m² /g. Frequently, it has a specific surface area greater than about1.0 m² /g, and more frequently of at least about 3.0 m² /g, usuallygreater than about 3.2 m² /g, and preferably at least about 3.4 m² /g.

Generally, the present aluminum nitride powder in the present mixture,i.e. after the components have been mixed, usually by milling, has aspecific surface area which can range widely, and generally it ranges toabout 10 m² /g. Frequently, it ranges from greater than about 1.0 m² /gto about 10 m² /g, more frequently from about 3.2 m² /g to about 8.0 m²/g, and still more frequently from about 3.4 m² /g to about 6.0 m² /g,according to BET surface area measurement. Generally, for a givencomposition of a compact, the higher the surface area of the aluminumnitride, the lower is the sintering temperature required to produce asintered body of a given porosity.

In the present process, processing of the aluminum nitride powder into acompact is carried out in air or at least partly carried out in air.During such processing, the aluminum nitride powder picks up oxygen fromair and any such pick up of oxygen is controllable and reproducible ordoes not differ significantly if carried out under the same conditions.By processing of the aluminum nitride powder into a compact, it is meantherein to include all mixing of the aluminum nitride powder to producethe present mixture, all shaping of the resulting mixture to produce thecompact, as well as handling and storing of the compact before it isdeoxidized by the additive.

In the present processing of aluminum nitride, the oxygen it picks upcan be in any form, i.e., it initially may be oxygen, or initially itmay be in some other form, such as, for example, water. The total amountof oxygen picked up by aluminum nitride from air other media generallyranges from greater than about 0.03% by weight to less than about 3.00%by weight, and usually it ranges from about 0.10% by weight to about1.00% by weight, and preferably from about 0.15% by weight to about0.70% by weight, of the total weight of the aluminum nitride. Generally,the aluminum nitride in the present mixture and compact prior tosintering has an oxygen content ranging from greater than about 1.0% byweight or from greater than about 1.85% by weight to less than about4.50% by weight, usually from about 2.00% by weight to about 4.00% byweight, and preferably it ranges from about 2.20% by weight to about3.50% by weight, of the total weight of aluminum nitride.

The present fluoride additive is a rare earth metal fluoride. As usedherein, the term rare earth metal fluoride includes yttrium fluoride,YF₃, and scandium fluoride, ScF₃, as well as the lanthanide fluorides,LF₃. Specifically, the present fluoride additive is selected from thegroup consisting of YF₃, ScF₃, LF₃ where L is La, Ce, Pr, Nd, Sm, Gd,Dy, Ho and Er, and a mixture thereof. YF₃ is useful in an amount rangingfrom about 0.5% by weight to about 15% by weight, preferably from about5% by weight to about 12% by weight, and more preferably from about 9%by weight to about 12% by weight, based on the amount of aluminumnitride powder. ScF₃ is useful in an amount ranging from about 0.5% byweight to about 10% by weight, preferably from about 3% by weight toabout 8% by weight, and more preferably from about 5% by weight to about8% by weight, based on the amount of aluminum nitride powder. LF₃ isuseful in an amount ranging from about 1% by weight to about 20% byweight, preferably from about 7% by weight to about 16% by weight, andmore preferably from about 12% by weight to about 16% by weight, basedon the amount of aluminum nitride powder. A mixture of any of thepresent fluoride additives is also useful in an amount ranging fromgreater than about 0.5% by weight to less than about 20% by weight basedon the amount of aluminum nitride powder. Amounts of the presentfluoride additive lower than the minimum given amount are not effectivefor producing the present sintered body. Amounts of the present fluorideadditive higher than the maximum given amount provide no advantage andmay lower the thermal conductivity of the sintered body due to theformation of too large a quantity of a second phase.

The particular amount of the present rare earth fluoride additive usedis determinable empirically and depends on such factors as the oxygencontent of the aluminum nitride powder, its specific surface area,sintering temperature and the desired density and thermal conductivityof the sintered body as well as the specific additive used.

In a given system, an increasing amount of oxygen in the aluminumnitride powder generally requires an increasing amount of the presentfluoride additive to deoxidize the aluminum nitride powder sufficientlyto produce the present sintered body.

Also, in a given system, generally as the specific surface area of thealuminum nitride powder is increased, the sinterability of the compactimproves resulting in a sintered body of higher density. This shouldalso enable the use of lower sintering temperatures.

In carrying out the present process, at least a substantially orsignificantly uniform mixture of the aluminum nitride powder and rareearth additive is formed and such mixture can be formed by a number oftechniques. Preferably, the powders are ball milled in a liquid mediumat ambient pressure and temperature to produce a uniform orsignificantly uniform dispersion. The milling media, which usually arein the form of cylinders or balls, should have no significantdeleterious effect on the powders, and preferably, they are comprised ofpolycrystalline aluminum nitride. The liquid medium should have nosignificantly deleterious effect on the powders and preferably it isnon-aqueous. Preferably, the liquid mixing or milling medium can beevaporated away completely at a temperature ranging from above room orambient temperature to below 300° C. leaving the present mixture.Preferably, the liquid mixing medium is an organic liquid such asheptane or hexane. Also, preferably, the liquid milling medium containsa dispersant for the aluminum nitride powder thereby producing a uniformor significantly uniform mixture in a significantly shorter period ofmilling time. Such dispersant should be used in a dispersing amount andit should evaporate or decompose and evaporate away completely or leaveno significant residue, i.e., no residue which has a significantdeleterious effect in the present process, at an elevated temperaturebelow 400° C. Generally, the amount of such dispersant ranges from about0.1% by weight to less than about 3% by weight of the aluminum nitridepowder, and generally it is an organic liquid, preferably oleic acid.

The liquid dispersion can be dried by a number of conventionaltechniques to remove or evaporate away the liquid and produce thepresent particulate mixture. If desired, drying can be carried out inair. Drying of a milled liquid dispersion in air causes the aluminumnitride to pick up oxygen and, when carried out under the sameconditions, such oxygen pick up is reproducible or does not differsignificantly. Also, if desired, the dispersion can be spray dried.

Generally, the fluoride additive in the present mixture has a specificsurface area which can range widely. Generally, it is greater than about0.4 m² /g and frequently it ranges from greater than about 0.4 m² /g toabout 6.0 m² /g, usually from about 1.0 m² /g to about 5.0 m² /g.

Shaping of the present mixture into a compact can be carried out by anumber of techniques such as extrusion, injection molding, die pressing,isostatic pressing, slip casting, roll compaction or forming or tapecasting to produce the compact of desired shape. Any lubricants, bindersor similar shaping aid materials used to aid shaping of the mixtureshould have no significant deteriorating effect on the compact or thepresent resulting sintered body. Such shaping-aid materials arepreferably of the type which evaporate away on heating at relatively lowtemperatures, preferably below 400° C., leaving no significant residue.Preferably, after removal of the shaping aid materials, the compact hasa porosity of less than 60% and more preferably less than 50% to promotedensification during sintering.

In a compact, an aluminum nitride containing oxygen in an amount ofabout 4.5% by weight or more generally is not desirable.

In the present sintering, the additive reacts with the oxygen of thealuminum nitride powder producing a fluoride gas which vaporizes away.Using YF₃ as an example, it is believed that the following deoxidationreaction occurs wherein the oxygen content of the aluminum nitride isgiven as Al₂ O₃ :

    2YF.sub.3 +Al.sub.2 O.sub.3 →Y.sub.2 O.sub.3 +2AlF.sub.3(g) (1)

In the deoxidation effected by the present fluoride additive, aluminumfluoride is produced which is volatile at temperatures above 1300° C.and which vaporizes away in the present sintering step thereby removingthe fluorine component.

The Y₂ O₃ formed in situ combines with Al₂ O₃ to produce a secondpolycrystalline phase or phases which may be as follows:

    3Y.sub.2 O.sub.3 +5Al.sub.2 O.sub.3 →2Y.sub.3 Al.sub.5 O.sub.12 (2)

    or

    5Y.sub.2 O.sub.3 +6Al.sub.2 O.sub.3 →2Y.sub.3 Al.sub.5 O.sub.12 Y.sub.4 Al.sub.2 O.sub.9                                  (3)

    or

    3Y.sub.2 O.sub.3 +Al.sub.2 O.sub.3 →Y.sub.4 Al.sub.2 O.sub.9 +Y.sub.2 O.sub.3                                          (4)

The specific amount of fluoride additive required to produce the presentsintered body can be determined by a number of techniques. It can bedetermined empirically. Preferably, an initial approximate amount ofadditive is calculated from Equation (1), that is the stoichiometricamount for YF₃ set forth in Equation (1), and using such approximateamount, the specific amount of YF₃ required in the present process toproduce the present sintered body would require one or a few runs todetermine if too little YF₃ had been added, or to optimize the amount ofYF₃. Specifically, this can be done by determining the porosity of thesintered body and its thermal conductivity.

The present compact is densified i.e., liquidphase sintered, at atemperature which is a sintering temperature for the composition of thecompact to produce the present polycrystalline body having a porosity ofless than about 10% by volume of the sintered body. Generally, theminimum sintering temperature is about 1800° C., but when GdF₃ is usedas the additive, it is about 1700° C. It is possible that when one ofthe other lanthanide fluoride is used as the additive, the minimumsintering temperature may be lower than about 1800° C., and such minimumsintering temperature would be determinable empirically. In the presentinvention, for a compact having a given amount of the rare earthcomponent, the minimum sintering temperature generally increases as theoxygen content of the aluminum nitride decreases. Minimum sinteringtemperature is dependent most strongly on composition and less stronglyon particle size of the aluminum nitride and the green density of thecompact, i.e., the porosity of the compact after removal of shaping aidmaterials. The present maximum sintering temperature is about 2050° C.

To carry out the present liquid phase sintering, the present compactshould contain a sufficient amount of the rare earth component, i.e., Y,Sc or L, as well as a sufficient amount of oxygen contained in thealuminum nitride powder to form a sufficient amount of liquid phase atsintering temperature to densify the compact to produce the presentsintered body. The present minimum densification, i.e., sintering,temperature depends mostly on the composition of the compact, i.e., theamount of liquid phase it generates. Specifically, for a sinteringtemperature to be operable in the present invention, it must generate atleast sufficient liquid phase in the particular composition of thecompact to carry out the present liquid phase sintering to produce thepresent product. A sintering temperature higher than about 2050° C.provides no significant advantage. Frequently, the present sinteringtemperature ranges from about 1800° C. to about 2000° C., preferablyfrom about 1800° C. to about 1950° C., and more preferably from about1850° C. to about 1950° C.

The compact is sintered, preferably at ambient pressure, in a gaseousnitrogen-containing nonoxidizing atmosphere which contains at leastsufficient nitrogen to prevent significant weight loss of aluminumnitride. In accordance with the present invention, nitrogen is anecessary component of the sintering atmosphere to prevent anysignificant weight loss of AlN during sintering. Significant weight lossof the aluminum nitride can vary depending on its surface area to volumeratio, i.e., depending on the form of the body, for example, whether itis in the form of a thin or thick tape. As a result, generally,significant weight loss of aluminum nitride ranges from in excess ofabout 5% by weight to in excess of about 10% by weight of the aluminumnitride. Preferably, the nitrogen-containing atmosphere is nitrogen, orit is a mixture of at least about 25% by volume nitrogen and a gasselected from the group consisting of hydrogen, a noble gas such asargon and mixtures thereof. Also, preferably, the nitrogen-containingatmosphere is comprised of a mixture of nitrogen and hydrogen,especially a mixture containing up to about 5% by volume, preferablyabout 2% by volume, of hydrogen.

Sintering time is determinable empirically. Typically, sintering timeranges from about 40 minutes to about 90 minutes.

The polycrystalline aluminum nitride body produced by the presentprocess is comprised of a polycrystalline aluminum nitride phase, i.e.,the primary phase, and a second polycrystalline rare earth-containingphase. The composition of this second phase can vary depending largelyon the composition of the compact, i.e., the unsintered body.Specifically, the second phase can be comprised of a rare earthaluminate phase or phases, a rare earth oxide phase or a mixture ofthese phases.

The amount of the polycrystalline second phase can vary dependinglargely on the amount of fluoride additive used and the oxygen contentof the aluminum nitride.

When YF₃ is the additive, the sintered body will contain anyttrium-containing phase generally in an amount ranging from about 0.1%by volume to about 10% by volume, preferably from about 0.1% by volumeto about 6% by volume, and more preferably from about 0.1% by volume toabout 3% by volume of the sintered body. The yttrium-containing phasegenerally is Y₃ Al₅ O₁₂, Y₄ Al₂ O₉, Y₂ O₃ or a mixture thereof.

When ScF₃ is the additive, the sintered body will contain ascandium-containing phase generally in an amount ranging from about 0.1%by volume to about 7% by volume, preferably from about 0.1% by volume toabout 5% by volume, and more preferably from about 0.1% by volume toabout 3% by volume of the sintered body.

When LF₃ is the additive, the sintered body will contain alanthanide-containing phase generally in an amount ranging from about0.1% by volume to about 6% by volume, preferably from about 0.1% byvolume to about 4% by volume, and more preferably from about 0.1% byvolume to about 3% by volume of the sintered body.

The present sintered polycrystalline body is a pressureless sinteredceramic body. By pressureless sintering herein it is meant thedensification or consolidation of the compact without the application ofmechanical pressure in the sintering step into a ceramic body having aporosity of less than about 10% by volume.

The polycrystalline body of the present invention is liquid-phasesintered, i.e., it sinters due to the presence of a liquid phase, thatis liquid at the sintering temperature and is rich in the rare earthcomponent and oxygen and contains some aluminum and possibly nitrogen.In the present polycrystalline body, the aluminum nitride grains haveabout the same dimensions in all directions, and are not elongated ordisk shaped. Generally, the aluminum nitride in the presentpolycrystalline body has an average grain size ranging from about 1micron to about 20 microns. An intergranular second phase of a rareearth aluminate is present along some of the aluminum nitride grainboundaries. The morphology of the microstructure of the present sinteredbody indicates that this intergranular second phase was a liquid at thesintering temperature.

The present sintered body has a porosity of less than about 10% byvolume, preferably less than about 4% by volume, more preferably, lessthan about 2%, and most preferably less than about 1% by volume of thesintered body. Any pores in the sintered body are fine sized, andgenerally they are less than about 1 micron in diameter. porosity can bedetermined by standard metallographic procedures and by standard densitymeasurements.

The present process is a control process for producing sintered body ofaluminum nitride having a thermal conductivity of at least 0.5 W/cm·K at25° C. The thermal conductivity of the present polycrystalline body isless than that of a high purity single crystal of aluminum nitride whichis about 2.8 W/cm·K at 25° C. If the same procedure and conditions areused throughout the present process, the resulting sintered body as athermal conductivity and composition which is reproducible or does notdiffer significantly. Generally, thermal conductivity increases with adecrease in volume % of second phase, and for a given composition withincrease in sintering temperature.

In the present process, aluminum nitride picks up oxygen in acontrollable or substantially controllable manner. Specifically, if thesame procedure and conditions are used in the present process, theamount of oxygen picked up by aluminum nitride is reproducible or doesnot differ significantly. Also, the present rare earth fluoride does notpick up oxygen, or does not pick up any significant amount of oxygen,from air or other media in the present process. More specifically, thepresent rare earth fluoride does not pick up any amount of oxygen in anyform from the air or other media which would have any significant effecton the controllability or reproducibility of the present process. Anyoxygen which the additive might pick up in the present process is sosmall as to have no effect or no significant effect on the thermalconductivity of composition of the resulting sintered body.

In the present invention, oxygen content may be determined by neutronactivation analysis.

By ambient pressure herein, it is meant atmospheric or about atmosphericpressure.

By specific surface area or surface area of a powder herein, it is meantthe specific surface are according to BET surface are measurement.

The present invention makes it possible to fabricate simple, complexand/or hollow shaped polycrystalline aluminum nitride ceramic articlesdirectly. Specifically, the present sintered body can be produced in theform of a useful shaped article without machining or without anysignificant machining, such as a hollow shaped article for use as acontainer, a crucible, a thin walled tube, a long rod, a spherical body,a tape, substrate or carrier. It is useful as a sheath for temperaturesensors. It is especially useful as a substrate for a semiconductor suchas a silicon chip. The dimensions of the present sintered body differfrom those of the unsintered body, by the extent of shrinkage, i.e.,densification, which occurs during sintering.

The present ceramic body has a number of uses. In the form of a thinflat piece, i.e., in the form of a substrate or tape, it is especiallyuseful as packaging for integrated circuits and as a substrate for anintegrated circuit, particularly as a substrate for a semiconducting Sichip for use in computers.

The invention is further illustrated by the following examples whereinthe procedure was as follows, unless otherwise stated:

The starting aluminum nitride powder contained oxygen in an amount ofabout 2% by weight and had a specific surface area of about 5 m² /g.

The starting aluminum nitride powder was greater than 99% pure AlNexclusive of oxygen.

The rare earth fluoride additive, before any mixing, had a specificsurface area of approximately 0.1 m² /g or greater (powders generally-325 mesh or finer).

Non-aqueous heptane was used to carry out the mixing, i.e. milling, ofthe powders. In some of the examples, oleic acid, or dioctyl phthalate,or a commercially available dispersant comprised of polyoxypropylene andpolyoxyethylene, was added to the heptane in an amount of about 1/2% byweight of the particulate mixture. The milling media was tungstencarbide in the approximate form of balls having a density of about 100%.

In Examples 1-3 the aluminum nitride powder alone, and in the remainingexamples the aluminum nitride and rare earth fluoride powders, wereimmersed in the liquid milling medium in a plastic jar and vibratorymilled in the closed jar at room temperature for the given period oftime.

The milled liquid dispersion of the aluminum nitride powder or givenpowder mixture was dried at ambient pressure in air at ambienttemperature or under a heat lamp under a cover of nitrogen.

In Example 2, after being milled for 7 hours, the aluminum nitridepowder had a specific surface area of about 6 m² /g, and in Example 3,after 16 hours of milling, its specific surface area was about 8 m² /g.

The dried milled powder in Examples 1-3, or dried powder mixture inExamples 4-26, was die pressed typically at about 5 Kpsi in air at roomtemperature to produce a compact having a density of roughly 55% to 60%of its theoretical density.

Each compact was in the form of a disk of substantially uniformthickness ranging from about 1/4 inch to about 1 inch. The disk had adiameter of about 3/8 inch or about 5/8 inch.

The furnace was a tungsten heater element furnace.

In all of the examples, the compacts were placed on a tungsten platebefore firing.

All compacts were fired in an atmosphere of nitrogen and held at thegiven sintering temperature for 1 hour.

The firing atmosphere was at ambient pressure, i.e. atmospheric or aboutatmospheric pressure.

At the completion of firing, the samples were furnace-cooled to aboutroom temperature.

All of the examples of Table I were carried out in substantially thesame manner except as indicated in Table I and except as indicatedherein.

Density of the sintered body was determined by the Archimedes method.

Porosity in % by volume of the sintered body was determined by knowingthe theoretical density of the sintered body on the basis of itscomposition and comparing that to the density measured using thefollowing equation: ##EQU1##

Phase composition of the sintered body was determined by opticalmicroscopy and/or X-ray diffraction analysis.

Based on the predetermined oxygen content of the starting AlN powder andthe measured compositions of some of the resulting sintered bodies, aswell as other experiments, it was calculated or estimated that in everyexample in Table I, the aluminum nitride in the compact before sinteringhad an oxygen content of roughly about 0.3% by weight higher than thatof the starting aluminum nitride powder.

The thermal conductivity of the sintered body of the examples, exceptExamples 9, 10, 13, 21, 22 and 24 was measured at 25° C. by a steadystate heat-flow method using a rodshaped sample ˜0.4 cm×0.4 cm×2.2 cmsectioned from the sintered body. This method was originally devised byA. Berget in 1888 and is described in an article by G. A. Slack in the"Encyclopedic Dictionary of Physics", Ed. by J. Thewlis, Pergamon,Oxford, 1961. In this technique the sample is placed inside ahigh-vacuum chamber, heat is supplied at one end by an electricalheater, and the temperatures are measured with fine-wire thermocouples.The sample is surrounded by a guard cylinder. The absolute accuracy isabout ±3% and the repeatability is about ±1%. As a comparison, thethermal conductivity of an Al₂ O₃ single crystal was measured with asimilar apparatus to be 0.44 W/cm·K at about 22° C.

The thermal conductivity of the sintered body of Examples 9, 10, 13, 21,22 and 24 was measured by laser flash at about 25° C.

The examples are illustrated in Table I.

Additive wt % in Table I shows the weight % of the given additive usedin Examples 4-26 based on the amount of aluminum nitride powder.Examples 1-3 were control samples, i.e. only aluminum nitride powder wasused to form the compact.

Heat-Up Time, hrs in Table I is the time it took to reach the givensintering temperature.

                                      TABLE 1    __________________________________________________________________________                                   Properties of Sintered Body                                   Sintered                                         Approximate     Thermal         Additive               Milling Time,                       Heat-up                             Sintering                                   Density                                         Porosity                                                 Second  Conductivity    Example         wt %  hrs.    Time/hrs                             T, °C.                                   g/cm.sup.3                                         vol %   Phases  W/cm. K at                                                         25° C.    __________________________________________________________________________    1    None  4       1     1850  2.64  19      --      --    2    None  7       1.5   1900  2.73  16      --      --    3    None  16      1     1975  3.10  5       --      0.4    4    0.5 YF.sub.3               4       1     1700  2.84  13      --      --    5    0.5 YF.sub.3               4       1.5   1800  3.26  0       --      --    6    1 YF.sub.3               4       1.5   1700  2.49  24      --      --    7    1 YF.sub.3               4       1.5   1800  3.26  0       --      --    8    1 YF.sub.3               4       1     1850  3.26  0       Y.sub.3 Al.sub.5 O.sub.12    9    1 YF.sub.3               7       1.5   1900  3.23  1       --      0.6    10   2 YF.sub.3               7       3     1900  3.24  2       --      0.8    11   5 YF.sub.3               4       1.5   1700  2.37  28      --      --    12   5 YF.sub.3               4       3     1850  3.22  3       Y.sub.4 Al.sub.2 O.sub.9                                                         --d                                                 Y.sub.3 Al.sub.5 O.sub.12    13   5 YF.sub.3               7       3     1900  3.28  1       --      1.0    14   9 YF.sub.3               4       3     1850  3.27  2       --      1.2    15   9 YF.sub.3               7       3     1900  3.31  1       --      1.4    16   12 YF.sub.3               7       4     1900  3.22  4       --      1.5    17   15 YF.sub.3               4       2     1825  3.10  9       Y.sub.4 Al.sub.2 O.sub.9                                                         1.3                                                 Y.sub.2 O.sub.3    18   20 YF.sub.3               4       5.5   1900  2.49  27      --      --    19    1 GdF.sub.3               7       1     1700  3.24  1       --      --    20    1 GdF.sub.3               7       1.5   1800  3.27  0       AlGdO.sub.3                                                         --    21    1 GdF.sub.3               7       1.5   1900  3.28  0       --      0.7    22   10 GdF.sub.3               7       3     1900  3.23  5       --      1.2    23   15 GdF.sub.3               7       3     1850  3.43  2       Gd.sub. 4 Al.sub.2 O.sub.9                                                 and     --                                                 Gd.sub.2 O.sub.3    24   15 GdF.sub.3               7       3     1900  3.37  3       --      1.2    25   20 GdF.sub.3               7       2     1900  3.31  7       Gd.sub.4 Al.sub.2 O.sub.9                                                 and     --                                                 Gd.sub.2 O.sub.3    26   20 ErF.sub.3               7       2     1900  3.37  6       Er.sub.4 Al.sub.2 O.sub.3                                                 and     --                                                 Er.sub.2 O.sub.3    __________________________________________________________________________

Examples 5, 7-10, 12-17 and 19-26 illustrate the present invention.

From a comparison of Examples 5, 7-10 and 12-17 and other work, it isknown that the thermal conductivities of the sintered bodies produced inExamples 5, 7, 8 and 12 would be at least 0.5 W/cm·K at 25° C.

From a comparison of Examples 19-26 and from other work, it is knownthat the thermal conductivity of the sintered bodies produced inExamples 19, 20, 23, 25 and 26 would be at least 0.5 W/cm·K at 25° C.

In copending U.S. patent application Ser. No. 880,516 entitled "AlkalineEarth Fluoride Additive For Sintering Aluminum Nitride" filed on June30, 1986 in the names of Stephen Lee Dole et al., assigned to theassignee hereof and incorporated herein by reference, there is disclosedthe production of an aluminum nitride ceramic body with a thermalconductivity of at least 0.5 W/cm·K at 25° C. by shaping a particulatemixture of aluminum nitride powder and an additive selected from thegroup consisting of CaF₂, SrF₂, BaF₂ and mixtures thereof into a compactand liquid phase sintering the compact.

What is claimed is:
 1. A process for producing a sinteredpolycrystalline aluminum nitride body having a porosity of less thanabout 10% by volume and a thermal conductivity of at least 0.5 W/cm·K at25° C. which consists essentially of forming a mixture consistingessentially of aluminum nitride powder containing oxygen ranging inamount from greater than about 1.0% by weight to less than about 4.0% byweight and YF₃, said YF₃ ranging from about 0.5% by weight to about 15%by weight based on the amount of said aluminum nitride powder, shapingsaid mixture into a compact, the aluminum nitride in said compactcontaining oxygen in an amount ranging from greater than about 1.0% byweight to less than about 4.5% by weight, and sintering said compact ina nitrogen-containing nonoxidizing atmosphere at a temperature rangingfrom about 1800° C. to about 2050° C. producing said polycrystallinebody, said nitrogen-containing atmosphere containing sufficient nitrogento prevent significant weight loss of said aluminum nitride.
 2. Theprocess according to claim 1 wherein said atmosphere is nitrogen.
 3. Theprocess according to claim 1 wherein said atmosphere is comprised of amixture of nitrogen and hydrogen, and said hydrogen ranges up to about5% by volume of said atmosphere.
 4. The process according to claim 1wherein said process is carried out at ambient pressure